Inositol 1,4,5-trisphosphate (InsP3) ligation of its receptor (InsP3R) results in release of stored intracellular calcium. This store-depletion couples to subsequent calcium influx via the store-operated calcium current Icrac. Calcium, derived from both intracellular store release and influx via Icrac, regulates a remarkably diverse range of cellular processes including growth, differentiation, death, and the acute effector responses of immune system cells to antigen. InsP3 levels are tightly controlled, and calcium release and influx responses can be dissociated by their differential sensitivity to cytosolic InsP3 levels. Modulation of either of these calcium signals for therapeutic purposes may be achieved via manipulation of the threshold for their generation. Here we propose to investigate the mechanisms that set the response threshold for InsP3-driven calcium release and influx. We hypothesize that both intrinsic properties of the InsP3R, and the environment provided by enzymes that metabolize InsP3, may be critical determinants of the specific activity of a given concentration of InsP3. Moreover, we will investigate the contribution of these factors to the generation of specific calcium-store subcompartments with different response thresholds, such as the calcium store that couples to Icrac activation (the CRAC store). This compartment is notable for its low sensitivity (i.e., high threshold) to InsP3. Finally, we will explore the biology of phosphoinositides other than Ins (1,4,5) P3 in the control of calcium release and influx responses in immune system cells.